1. Field of the Invention
The present invention relates to an image processing method and apparatus suitable for a facsimile machine, a digital copying machine, and the like, which use, e.g., recycle paper as originals.
2. Description of the Prior Art
In a conventional image processing apparatus such as a facsimile machine, an output value from a Laplacian filter shown in FIG. 20, or a value obtained by weighting an output value obtained by changing the coefficient of the Laplacian filter is added to a pixel of interest as a pixel to be processed, thereby emphasizing the edge of an image.
However, in the prior art, in the edge emphasis processing, the degree of emphasis is increased for a region having a higher spatial frequency. For this reason, when recycle paper popularly used in recent years is read, the noise components on an original are undesirably emphasized. Thus, when the read image is recorded, image quality is deteriorated.
In order to suppress emphasis of the noise components, if no edge emphasis is performed or the degree of emphasis is lowered, the edge portion of a character or figure is blurred, and reproducibility of thin lines is impaired.
On the other hand, when an image on an original having a background color such as newspaper or blue print is to be read, a conventional image processing apparatus performs background noise reduction by a method for forming a density histogram by, e.g., a pre-scan operation, and determining a threshold value upon binary conversion of the read image on the basis of the density histogram, or by processing for detecting an average value of image densities in units of blocks each consisting of a plurality of pixels, and dynamically varying the threshold value upon binary conversion (so-called ABC: Automatic Background Control processing).
In recent years, recycle paper has received a lot of attention for the purpose of effective use of natural resources. However, recycle paper includes many unique granular noise components as compared to high-quality paper. For example, when a conventional facsimile machine reads an image on recycle paper, and binary-converts the read image, granular noise appears on the background, and problems of deterioration of image quality, a decrease in compression ratio upon encoding, and the like are posed.
Even when the above-mentioned ABC (Automatic Background Control) processing is performed for such a recycle paper original, granular noise having a high density cannot be satisfactorily removed. In order to avoid the influence of granular noise, when resolution correction processing (so-called edge emphasis processing) is inhibited, the resolution of the read image is decreased. When the binary-conversion threshold value is changed, an original image having a low density level cannot be reproduced.